The drainage basin & water cycle

A drainage basin is the area of land drained by a river and its tributaries. It is an open system — a working part of the water cycle, with inputs, stores, transfers and outputs.

• Input: precipitation (rain, snow, hail).
• Store: interception — water held on leaves and plants.
• Transfers (flows): surface runoff (overland flow), infiltration (water soaking into soil), throughflow, percolation (into rock) and groundwater flow.
• Outputs: river discharge into the sea, and evapotranspiration (evaporation + transpiration) back to the air.

Key basin terms: the watershed is the high boundary of the basin; the source is where the river begins; a tributary is a smaller river joining the main one at a confluence; the river mouth is where it meets the sea.

Name the boundary

How a river changes downstream

Follow a river from source to mouth and its characteristics change along the long profile:

• Gradient is steep near the source and gets gentler downstream.
• Width and depth both increase downstream.
• Discharge (the volume of water passing a point per second) increases as tributaries add water.
• Load becomes smaller and rounder downstream as it is worn down.

Erosion & transportation

Rivers do three kinds of work: they erode (wear away), transport (carry) and deposit (drop) material called the load.

Four erosion processes:

• Hydraulic action — the sheer force of moving water forcing air into cracks.
• Abrasion (corrasion) — the load scrapes and grinds against the bed and banks.
• Attrition — load particles knock together and become smaller and rounder.
• Solution (corrosion) — mildly acidic water dissolves soluble rock such as limestone.

Four transport processes: solution (dissolved), suspension (fine sediment carried in the flow), saltation (small stones bounced along) and traction (large boulders rolled along the bed).

Erosion, transport or deposition?

Tap a process, then tap the group it belongs to.

Waterfalls

A waterfall forms where a band of hard rock lies over softer rock. The softer rock is eroded faster by hydraulic action and abrasion, undercutting the hard rock and forming a plunge pool below.

Meanders

In the middle and lower course a river swings in bends called meanders. Water flows fastest on the outside of the bend and slowest on the inside:

• Outside bend — fast flow erodes the bank, forming a steep river cliff.
• Inside bend — slow flow deposits sediment, building a gentle slip-off slope.

Inside or outside?

Floodplains & levees

The floodplain is the wide, flat valley floor either side of a river in its lower course. When a river floods, it spreads out, slows down and deposits its load:

• Repeated floods lay down fine alluvium, building a flat, fertile floodplain.
• The heaviest material is dropped first at the banks, building up raised ridges called levees.

Levees can form naturally, but people also build them up as flood walls to raise the channel's capacity.

Why do rivers flood?

Flooding happens when discharge exceeds the channel's capacity and water spills onto the floodplain. Causes are physical and human:

• Physical: prolonged or very heavy rainfall; rapid snowmelt; steep slopes; impermeable rock or saturated soil that stops infiltration.
• Human: deforestation (less interception), urbanisation (tarmac and drains speed runoff to the river).

Case study flag: CCEA requires one flooding case study from the British Isles. A commonly taught example is the Somerset Levels floods of 2014. Check the specific figures (rainfall totals, homes flooded, cost) against your own class case-study notes before using them in an exam answer.

Your turn — river discharge

Managing flooding — hard vs soft

Flood management uses two broad approaches. Hard engineering builds structures to control the river; soft engineering works with natural processes and is usually more sustainable.

• Hard engineering: dams, flood walls, embankments/levees, and straightening & deepening the channel to move water away faster.
• Soft engineering: washlands (areas allowed to flood), land-use zoning (keeping housing off the floodplain) and afforestation (planting trees to increase interception).

Evaluation: CCEA asks you to evaluate a river management strategy outside the British Isles (a common example is the Mississippi). Hard schemes are effective but costly and can shift flooding downstream; soft schemes are cheaper and greener but need more land. Confirm case-study specifics against your class notes.

Match each method to its type

Tap a method on the left, then its matching type on the right.

Which is the most sustainable?

Your turn — reading flood data

The big ideas to know

You've covered all of CCEA Theme A — River Environments. Press Finish to see your score.